What is listed for armor class in that equipment guide us what the number by the shield should be if you wear that armor......lower numbers are better for that. The armor class number listed in got inventory is how much is subtracted from your naked(boxer shorts) value of 10.....the modifiers add or subtract from the inventory AC......+ modifiers are improvements which result in lower numbers by that shield......and yes, you can get negative numbers up there......means it's much harder for most monsters to hit you.

Aquators RUST your armor, as do rust traps......NOT a good thing at all

Yeah the Armor Class (AC) is a holdover from ye olde days of Dungeons & Dragons and can be confusing for those not familiar with it.Just remember lower=better for armor, anything else higher=better (unless it's monsters)